The present invention relates to wireless communications, and more particularly, to methods and controllers that manage Hybrid Automatic Repeat reQuest (HARQ) processes to handle transmission errors regarding the HARQ transmission.
To improve the reliability of data transmission, HARQ scheme that is a combination of Forward Error Correction (FEC) and Automatic Repeat-reQuest (ARQ) is added to packet data transmission systems. In HARQ scheme, error correction codes sent by a transmitting end can be used to detect errors as well as correct errors. After a receiving end receives packets and error correction codes, if an error exists and is correctable with respect to error correction codes sent by the transmitting end, the error will automatically be corrected; otherwise, the receiving end will send corresponding feedback signals to the transmitting end, asking for a retransmission. Please refer to FIG. 1 illustrating an example of a conventional wireless communications system 100 including HARQ scheme conforming to wideband code division multiple access (W-CDMA) standard. The wireless communications system 100 includes a user equipment (UE) 110, a Node B 120 and a radio network controller (RNC) 130. In this system, a protocol data unit (PDU) in an uplink scenario is sent from a transmitting end comprising the user equipment 110 to a receiving end comprising the Node B 120 and the RNC 130. In a downlink scenario, the transmitting end comprises the Node B 120 and the RNC 130, sending the PDU to the UE 110. For transmission on a medium access control (MAC) layer of the wireless communications system 100, the MAC PDU fails to be received could be retransmitted with a HARQ process.
HARQ entities are located in a medium access control (MAC) layer 112 of the UE 110 as well as a MAC layer 122 of the Node B 120. The HARQ entity of the UE 110 can include eight HARQ processes, wherein different HARQ processes are identified by process IDs. The HARQ entity of the UE 110 and the HARQ entity in Node B 120 are peer entities, including the same number of HARQ processes; each HARQ process forms the one-to-one protocol entity through the process ID. The HARQ process on the Node B 120 receives the MAC PDU sent by the HARQ process on the UE 110. Unfortunately, there are problems that are related to transmission failures regarding the HARQ scheme, which significantly affects the performance of the wireless communications system 100.
Problem I:
In certain conditions, HARQ transmission failure may occur in some HARQ processes more frequently than in other HARQ processes. This imbalance between occurrences of HARQ transmission failures on different HARQ processes leads to certain problems when some important data are sent via these HARQ processes. Please refer to FIG. 2, which illustrates the imbalance between occurrences of the HARQ transmission failures on the HARQ processes. In the table shown in FIG. 2, the “Avg. retransmission count” means how many times of retransmissions are required by a HARQ process to successfully transmit a PDU. In the condition represented by the table, HARQ process 1 and HARQ process 7 require considerably more retransmissions for successfully transmitting a PDU while HARQ processes 0, 2 and 4 require relatively fewer retransmissions for successfully transmitting a PDU. That is, HARQ transmission failure occurs more frequently in HARQ process 1 and HARQ process 7 than in other HARQ processes. If a control PDU or a retransmission PDU from an upper layer (e.g. RLC layer) is mapped to and transmitted via HARQ process 1 and HARQ process 7, the latency of the wireless communications system 100 will be deteriorated.
Problem II:
When a MAC PDU is correctly received by a HARQ process of the MAC layer 122 of the Node B 120 (the receiving end), the HARQ process of the MAC layer 122 of the Node B 120 sends a positive acknowledgment (ACK) response back to a corresponding HARQ process of the MAC layer 112 of the UE 110. The correctly received MAC PDU will further sent from the MAC layer 122 of the Node B 120 to a radio link (RLC) layer 132 of the radio network controller (RNC) 130 in the wireless communications system 100.
However, the radio link control (RLC) layer 132 of the RNC 130 may fail to correctly receive the MAC PDU sent by the MAC layer 122 of the Node B 120. Alternatively, the RLC layer 132 of the RNC 130 may lose RLC packets due to loading issues even if the RLC packets have been correctly received by Node B 120.
Once the above conditions occur, the RLC layer 132 of the RNC 130 signals the RLC layer 114 of the UE 110 with a negative acknowledgment (NACK) response. This may be caused by poor network status between the Node B 120 and RNC 130. Such transmission failure is referred to as HARQ ACK-to-RLC NACK error, which further gives rise to RLC retransmission between the UE 110 and Node B 120. As it takes pretty long time to request the RLC layer 114 of the UE 110 to retransmit RLC PDU, this severely reduces the performance of the wireless communications system 100.